A single 15-minute session of high-intensity exercise masked symptoms of subsequent hypoglycemia in patients with type 1 diabetes and normal hypoglycemia awareness, according to Dutch investigators.
Twenty patients with type 1 diabetes – 10 with normal hypoglycemia awareness and 10 with impaired awareness – and 10 healthy subjects were asked to push themselves as hard as they could for 30-second bursts during 15 minutes of stationary bicycling and to go at an easier pace in between, reported Hanne Rooijackers, MD, of the Radboud University Medical Center, Nijmegen, the Netherlands, and colleagues.
The subjects had intravenous cannulae in both forearms, one for blood draws and the other for glucose and insulin administration. All three groups were kept euglycemic during exercise, but then were subjected to hyperinsulinemic-hypoglycemic clamps afterwards. Plasma glucose levels were allowed to fall to 2.6 mmol/L over about 35 min., and were kept there for another 60 min. Trembling, palpitations, anxiety, and other symptoms were serially assessed while patients were hypoglycemic, along with cognitive function and levels of hormones involved with hypoglycemic defense.
For comparison, the subjects all had clamps applied and hypoglycemia symptoms and physiologic responses assessed after a 15-minute session of rest at least 2 weeks apart from the exercise session.
The healthy subjects had a peak of about 20 points on a composite score of hypoglycemia symptoms during rest; exercise reduced the peak score only a small amount to about 18 points. Diabetic patients with normal hypoglycemia awareness hit a peak symptom score of 31 points during rest, which fell substantially after exercise to 22 points. Exercise, meanwhile, had no effect on diabetic patients with impaired awareness; after both rest and exercise, they had a peak composite symptom score of about 11 points, Dr. Rooijackers and colleagues reported (Diabetes. 2017 Jul;66[7]:1990-8).
High-intensity interval training (HIIT) did not affect hypoglycemic awareness in patients with impaired awareness probably because of “a ‘floor’ effect, in that symptom responses could not be further suppressed than they already were,” the investigators speculated.
Regular exercise is recommended for patients with type 1 diabetes, and, like others, they are turning to HIIT – short bursts of intense exercise broken up by brief periods of rest or lower intensity movement – because it appears to deliver the benefits for more moderate exercise in less time.
The findings suggest, however, that high-intensity exercise might increase the risk of severe hypoglycemia in type 1 patients by reducing awareness of its symptoms and blunting hormonal defenses.
The team suspects elevated lactate levels account for the findings. Exercise increases plasma lactate levels, and as blood glucose levels fall, the brain uses lactate as an alternative fuel, which likely blunts the effects of hypoglycemia. Plasma lactate spiked in the study subjects about 15 minutes after exercise.
Participants presented early in the morning after fasting overnight and abstaining from strenuous exercise for 2 days. They were in their mid-20s on average, normal weight, and fairly well balanced between men and women. Patients with type 1 diabetes were eligible for the study if they had hemoglobin A1c levels below 9% and no vascular complications beyond retinopathy. Their duration of diabetes was about 10 years.
The work was funded by the Dutch Diabetes Research Foundation and the European Foundation for the Study of Diabetes. The authors had no conflicts of interest.